In an industrial field of transparent sheets, there has been widely used a polyvinylchloride sheet having good transparency and moldability as well as being inexpensive. On the other hand, in recent years, the use of the polyvinylchloride sheets has been being restricted due to increasing awareness of environmental issues, and there has been a demand for a new transparent sheet substituting for the polyvinylchloride sheet.
As a substitute for the polyvinylchloride sheet, a sheet made of a polypropylene has been attracting attentions. However, since the polypropylene is a crystalline resin, it might not provide sufficient transparency. Since the transparent sheet is typically used in an application for packaging various articles, the transparent sheet as a sheet base material is required to be excellent in various properties other than the transparency as a basic property.
As the various properties, the transparent sheet is required to exhibit high impact resistance especially at a low temperature and a little whitening in folding processing. As for the impact resistance at a low temperature, when, for instance, the transparent sheet is applied to folded boxes which are carried in a low-temperature area, a cushioning material has conventionally been used to prevent breakage of the folded boxes, which was very costly. Accordingly, there has been a strong demand for improving the impact resistance of the sheet itself at a low temperature
Attempts have been made in various fields to improve the transparency and the impact resistance of a transparent polypropylene based sheet. For example, there has been proposed a method for obtaining a sheet using a so-called quenching method in which a polypropylene resin containing a homopolypropylene (HPP) and a low crystalline resin is melt-extruded and quenched.
Such technology may include: a method in which a polypropylene resin containing a homopolypropylene (HPP) and an ethylene-butene-1 copolymer is melt-extruded and then quenched to obtain a sheet (see, for instance, Patent Document 1); and a method using a polypropylene resin composition containing a homopolypropylene (HPP), a low density ethylene-butene-1 copolymer and a nucleating agent (see, for instance, Patent Document 2). Incidentally, as the latter method, there may also be known a method using a polypropylene resin containing a homopolypropylene (HPP) and a random polypropylene (RPP) (see, for instance, Patent Document 3).
There has also been proposed a method in which a homopolypropylene (HPP) containing a linear low-density polyethylene (L-LDPE) is used as a polypropylene resin, which is melt-extruded and then quenched (see, for instance, Patent Document 4); and a method in which a polypropylene resin composition containing a low crystalline polypropylene is quenched and heat-treated (see, for instance, Patent Document 5).
Meanwhile, as a method for improving the rigidity and the transparency of the transparent polypropylene based sheet, there has been known a method for obtaining a sheet in which a nucleating agent, an aliphatic or an alicyclic petroleum resin, etc. are added to a raw material of a polypropylene (see, for instance, Patent Document 6).
[Patent Document 1] JP-B-6-81698
[Patent Document 2] JP-B-6-81796
[Patent Document 3] JP-A-1-306448
[Patent Document 4] JP-A-9-29818
[Patent Document 5] JP-A-2003-170485
[Patent Document 6] JP-A-58-25341